Russian investigators today charged four men in connection with the murder of the investigative journalist Anna Politkovskaya, adding that the preliminary inquiry into her death was now "over".

The investigators' committee said it had charged three men with involvement in the killing of Politkovskaya, who was shot dead outside her Moscow apartment in October 2006.

It also charged an officer from the Federal Security Service (FSB) - Russia's post-KGB spy agency - with extortion and abuse of office, the committee said in a statement. All four have been in prison since August.

Today's developments, however, leave several major questions unanswered. The committee has so far apparently been unable to identify who ordered Poltikovskaya's execution.

Officials have publicly accused the 34-year-old Chechen Rustam Makhmudov, but so far he has eluded arrest with investigators saying he may have fled abroad.

Two of those charged today are Makmudov's brothers - Dzhabrail and Ibragim, also from Chechnya. The third suspect, Sergei Khadzhikurbanov, is a police officer.

The fourth man, Pavel Ryaguzov, a lieutenant colonel in the FSB, was charged in relation to other crimes. Officials have previously accused him of supplying the killers with Politkovskaya's address and other details.

Politkovskaya's colleagues at Novaya Gazeta, the small liberal newspaper where she worked, said they were deeply sceptical that the investigation had got to the bottom of her murder.

"The investigation isn't finished. Only part of the case is over," Sergei Sokolov, the paper's chief editor, told the Guardian. He added: "There is no doubt the investigation committee is working. But we need to wait for a court case until we can judge anyone's guilt."

Russia's chief prosecutor, Yuri Chaika, hinted last year that the mastermind behind the killing was Boris Berezovsky - the Kremlin critic and former oligarch who lives in London. Berezovsky has dismissed the claim as politically motivated and nonsense.

Novaya Gazeta has said that it is far more likely the person who ordered her killing lives in Russia. "There have been claims she was murdered by supporters of the Kremlin and opponents of the Kremlin. But this is just speculation. What we need is facts," Sokolov said.

Politkovskaya, 48, was gunned down in the lift to her apartment. A young man wearing a baseball cap was captured on CCTV trailing her in a supermarket. He shot her twice in the chest before administering a final coup de grace to the head.

Her murder sparked widespread international criticism. It was linked to her penetrating reportage from Chechnya, where she wrote about human rights abuses by Russian and Chechen forces during two brutal wars, and her fearless criticism of Vladimir Putin's regime.

Nine people were arrested last year in connection with her death. Until today, however, officials had quietly released five suspects, allegedly due to lack of evidence. Today the committee confirmed it was dropping charges against one of those released, Shamil Burayev, a regional Chechen official.

Politkovskaya's colleagues have accused the authorities of selectively leaking sensitive information to the media, with the apparent aim of allowing suspects, including the alleged killer, to escape.

"No criminal cases have been opened in relation to those who released leaks. It's through their fault the murderer remains at large," Dmitry Muratov, Noveya Gazeta's editor-in-chief, told Interfax today.

He said his paper would continue its own independent investigation into the murder, and collaborate with federal investigators. "We believe, as before, that the investigation is on the right track. But one cannot interpret this as an end, or say that the crime has been solved."